hghave: make black version regex work with newer versions of black

Black commit 117891878e5be4d6b771ae5de299e51b679cea27 (included in black >=
21.11b0) dropped the string "version " from the output of "black --version". To
make the regex work with newer black versions, make matching of "version "
optional.

tests/hghave.py

@@ -1123,7 +1123,7 b' def has_emacs():'
 @check('black', 'the black formatter for python (>= 20.8b1)')
 def has_black():
     blackcmd = 'black --version'
-    version_regex = b'black, version ([0-9a-b.]+)'
+    version_regex = b'black, (?:version )?([0-9a-b.]+)'
     version = matchoutput(blackcmd, version_regex)
     sv = distutils.version.StrictVersion
     return version and sv(_bytes2sys(version.group(1))) >= sv('20.8b1')